In recent years, there has been a demand for increasing the recording density of a rewritable disc such as a phase-change type optical disc or a magneto-optical disc in order to enable recording a large amount of data such as moving-video data without increasing the diameter of the disc. This increase in recording density is realized by making small of the spot size of a laser beam irradiating a signal-recording surface of the optical disc.
It is known that the spot size (d) in the order near the wavelength .lambda. of the laser beam, according to the Fourier image formation theory, can be determined from the wavelength .lambda. and the numerical aperture NA of the objective lens for focusing the laser beam by the following equation (1). EQU d=1.22.multidot..lambda./NA (1)
Accordingly, the shorter the wavelength of the laser beam is and the greater the numerical aperture of the objective lens is, the smaller the spot size becomes. This makes it possible to increase the recording density.
As a method of increasing the numerical aperture, there is known a method using a solid immersion lens.
Stating in terms of its principle, as illustrated in FIG. 1A, this method disposes, between an objective lens 61 and an optical disc 62, a solid immersion lens (SIL) 63 whose surfaces facing the objective lens 61 and optical disc 62 are a spherical surface 63a and a flat surface 63b, respectively and causes a laser beam L having passed through the objective lens 61 to vertically enter the spherical surface 63a of the SIL 63 and to focus on a central portion of the flat surface 63b. Assuming that n represents the refractive index of the SIL 63, the numerical aperture (effective numerical aperture) of a lens group consisting of the objective lens 61 and the SIL 63 becomes n times as great as the numerical aperture of the objective lens 61 itself.
However, the method is actually arranged to cause the effective numerical aperture to become as n.sup.2 times as great as that of the objective lens 61 by satisfying the conditions of stigmatic focusing as described below. To this end, as illustrated in FIG. 1B, the method causes the laser beam L that has passed through the objective lens 61 to enter the spherical surface 63a of the SIL 63 at an incidence angle different from the angle vertical to this spherical surface 63a, thereby causing the laser beam L to be somewhat refracted by the spherical surface 63a.
In, for example, S. M. Mansfield et al's thesis entitled "High-numerical-aperture lens system for optical storage", carried on pages 305 to 307 of "Optics Letter" the 18th issue, published in 1993 (hereinafter called "reference literature no. 1"), H. J. Mamin et al's thesis entitled "Near-field optical data storage", carried on pages 141 to 143 of "Applied Physics Letter" the 68th issue, published in 1996 (hereinafter called "reference literature no. 2"), it is reported that by the method using the solid immersion lens, the numerical aperture exceeding 1 is realized.
By the way, when the numerical aperture exceeds 1 in that way, as the distance (air gap) between the solid immersion lens and the optical disc in a direction along an optical axis of the laser beam increases, the reflectance of a component of the laser beam which corresponds to the optical quantity exceeding 1 of numerical aperture at a flat surface of the solid immersion lens increases. As a result of this phenomenon, the intensity of the laser beam permeating the solid immersion lens and irradiating the optical disc rapidly deteriorates. When this air gap becomes more than the range of the proximity field (nearfield), most of the component corresponding to the optical quantity exceeding 1 of numerical aperture becomes reflected by the flat surface of the solid immersion lens. Therefore, the intensity of the laser beam irradiating the optical disc becomes remarkably low.
In order to represent this specifically, FIG. 2 is made regarding each case for the air gap of 0 nm, 50 nm, 100 nm, 200 nm and 500 nm as follows. The distance on the signal-recording surface of the optical disc as measured from the center of the spot of the laser beam is plotted on the abscissa axis. In contrast, the intensity of the laser beam irradiating this signal-recording surface (as expressed in terms of the ratio to the intensity of the laser beam at the spot center thereof when the air gap is 0 nm) is plotted on the ordinate axis. FIG. 2 shows the calculated values of the intensity distribution (Strehl intensity) of the laser beam on the signal-recording surface when the numerical aperture NA=1.5 and the wavelength .lambda.=640 nm.
FIG. 2 shows the following. When the air gap is 50 nm, the intensity of the laser beam at the center of the spot is to an extent of 85% of the intensity exhibited when the air gap is 0 nm. However, when the air gap increases up to 100 nm, the resulting intensity becomes approximately 60% of the intensity when the air gap is 0 nm. When the air gap reaches 200 nm, the resulting intensity falls down to approximately 35% of the intensity when the air gap is 0 nm.
Therefore, where the numerical aperture exceeds 1, control should be performed so as to make the air gap sufficiently small (in the case of the example illustrated in FIG. 2, so as to keep the air gap fall within the range of 100 nm or less at the maximum, or preferably 50 nm or so). Otherwise, due to the fall in the intensity of the laser beam irradiating the signal-recording surface of the optical disc, the recording precision and the reproduction precision will disadvantageously deteriorate.
As the method of controlling so as to make the air gap small, there is also a method in which the optical head having an objective lens and solid immersion lens installed therein is floated from the optical disc by the air current involved by the rotation thereof as in the case of the magnetic head of a hard-disc device.
However, by this method, the intensity of the air current depends on the linear velocity of the optical disc, so that, for example, in the case of a CAV (constant-angular-velocity recording) system, as the irradiating position of the laser beam in the radial direction of the disc changes (i.e., as the track to be accessed changes), the amount of floating of the head inconveniently changes. In the case of a CLV (constant-linear-velocity recording) system also, the amount of floating differs between optical disc devices whose linear velocities differ from each other. As a result, this method is difficult to control the air gap with high precision.
In view of the above, the applicant of this patent application already filed in Japanese Patent Office (Japanese Patent Application Laid-Open No. 8-212579) a patent application for an invention concerning an optical head and a driving apparatus for an optical recording medium. This previous invention is arranged to retain an objective lens and a solid immersion lens by respective separate holders and use electrically conductive material in the holder for retaining the solid immersion lens, thereby causing the control of the position in the direction along an optical axis of the solid immersion lens in accordance with the electrostatic capacity (capacitor) formed with this electrically conductive material to be performed independently of the control of the distance in the direction along the optical axis between the objective lens and the optical disc. According to this previous invention, it is possible to control the air gap with high precision regardless of the linear velocity of the optical disc.
However, in this previous invention proposed by the present applicant, two actuators are necessary as an actuator for moving the lens in the optical-axial direction for the purpose of a focus servo. One is an actuator for moving the holder having the solid immersion lens retained thereby and the other is an actuator for moving the holder having the objective lens retained thereby.
Also, as the signal processing for producing a control signal for focus servo, two kinds of signal processing are necessary. One is the signal processing for producing a control signal that is intended to control the position of the solid immersion lens in accordance with the electrostatic capacity. The other is the signal processing for producing a control signal intended to control the distance between the objective lens and the optical disc (e.g. the matrix processing of the output signal of a photo-detector having received a laser beam reflected from the optical disc).
Therefore, the present invention has an object to provide an optical head and a driving apparatus for an optical recording medium, which are capable of making the numerical aperture greater with a solid immersion lens, controlling the air gap according to the electrostatic capacity with high precision, and also making simpler of the actuator and the signal processing for focus servo.